Drilling machines, especially those for use in producing printed circuit boards, are confronted with demands for ever-increasing rotational speeds. Conventional drill spindles, even those of the most advanced design, have been unable to achieve rotational speeds needed for achieving desired production rates, hole quality and drill life.
The problem of drill rotation speed is especially acute for drills of smaller sizes to produce the kinds of openings increasingly required in printed circuit boards. The machines are called upon to make use of drills of 8 mils diameter and under in producing printed circuit boards. Because of the small drill diameter, the surface speed at the cutting edge is relatively low with conventional spindles. Low surface speed of the drill results in a short drill life and reduced hole quality. Production rates also suffer.
In addition to their inability to rotate drills at optimum speeds, conventional drill spindles are unable to maintain the axes of the drills in precise alignment with the path of movement of the spindle as it reciprocates in the drilling operation. For a drill of relatively large size, a slight cocking is not of major consequence. However, small drills are delicate and are easily broken as a result of even slight angularity of the drill axis relative to the direction of the movement of the spindle. Consequently, in drilling with small drills, breakage of the drills has been a major problem. In an effort to combat this, the manufacturers of drills of small diameter have manufactured the drills of metal which is not as hard as that normally used in drill production, hence being less brittle. Although this may have to a degree reduced drill breakage, it has by no means solved the problem as high rates of drill breakage continue for small drills. Moreover the less brittle material is softer and wears faster, reducing drill life.
Conventional drill spindles are complex and expensive machines. Typically, the drill spindle includes an electric motor within a housing which must be water cooled. Hence, provision is made for circulating water through the spindle assembly. An automatic collet is part of the spindle to enable drill changes to be made. The entire spindle assembly must be reciprocated vertically in order to effect drilling. This requires a large and cumbersome mechanism. The rotor of the spindle must be mounted in precision bearings requiring a long run-in period and with a limited service life.